A flip chip semiconductor device refers to a packageless semiconductor device which is used for mounting to a substrate, such as a printed circuit (PC) board, in a die-down or chip-down position. In other words, an active surface of a semiconductor die which is a component of the device will be facing the substrate. Typically, a flip chip device is mounted to a substrate by forming a plurality of conductive bumps, for example solder bumps, on an active surface of a semiconductor die and electrically coupling these bumps to a corresponding pattern of electrical terminals on a substrate. A common method of coupling the bumps to the terminals is by positioning the active surface of the die adjacent a surface of the substrate such that the bumps align with the electrical terminals. The die and substrate are subjected to a heated environment such that the bump material begins to soften or flow, thereby wetting the electrical terminals. Upon cooling, the bump material hardens and forms a metallurgical bond between the bump on the die and the electrical terminal on the substrate. Flip chip die can also be electrically coupled to a substrate using a combination of solder bumps and solder balls with varying compositions in order to achieve eutectic solder joints.
An advantage in using flip chip technology is that device size can be kept to a minimum since the device does not employ a traditional package body. Furthermore, electrical connections between a semiconductor die and a substrate are confined to an area of the substrate which does not exceed the size of the die. There is no need for wire bonds or for any kind of external lead in order to couple the die to the substrate.
However, a substantial disadvantage in using flip chip technology is that there is not a manufacturable method of burning-in flip chip devices, primarily because such devices lack external leads. Burn-in refers to tests which many semiconductor manufacturers use to screen weak devices before shipping the devices to customers. A common burn-in procedure is to operate the devices at elevated temperatures and high voltages to detect early device failures. Because the devices are being operated during burn-in, the device must be electrically coupled to burn-in testing equipment. A widely accepted burn-in set-up involves affixing a plurality of similar semiconductor devices to a burn-in test board, usually by placing each device in a pre-established test socket which is attached to the board. The board is then electrically connected to the test equipment such that a plurality of devices can be burned-in simultaneously. Flip chip semiconductor devices are not able to be burned-in according to the above procedure because flip chip devices cannot be used in existing test sockets. Conventional test sockets are designed to accommodate external device leads which flip chip devices lack. Most flip chip devices have solder bumps on the active surface of the die so that ordinary test sockets cannot be used for burn-in testing of the devices. For this reason, many manufacturers have chosen not to perform burn-in testing on flip chip devices. As a result, some defective flip chip devices which would otherwise be identified as early failures during burn-in are being sent to customers.
Another disadvantage in using existing flip chip technology is that conductive bumps formed on an active surface of a semiconductor die often require an additional level of metallization to be formed on the die. After forming metallized bonding pads on the die, an additional metal layer is usually deposited and patterned in order to transform the bond pad configuration into a configuration which will match a substrate's electrical terminal configuration. In addition to processing steps related to the extra metal layer, flip chip devices also require at least one additional insulating layer and several masking operations. Adding processing steps in die fabrication increases the probability of creating a defect, thereby lowering device yield. Not only does the additional metal layer adversely impact device fabrication, but the presence of an additional metal layer may increase device capacitance which is undesirable.
Yet another disadvantage associated with flip chip technology is that the ability to perform rework on a device is often restricted. Upon mounting a flip chip device to a substrate, such as a PC board, many device users underfill the mounted device, or in other words fill the space between a semiconductor die and the substrate. Thermally conductive epoxy is one material of choice for flip chip underfill. The purpose in underfilling flip chip devices is to constrain the expansion and contraction of the semiconductor die with respect to the substrate. In general, a semiconductor die has a coefficient of thermal expansion which is quite different than a coefficient of the thermal expansion for the substrate. As a result, the die will expand and contract at a different rate than will the substrate, creating stress on solder joints and a potential for the electrical connections between the die bonding pads and the substrate terminal pads to become open. The use of an underfill material helps to constrain the expansion and contraction of the die, thereby reducing the potential for open connections. However, the use of an underfill material also prohibits rework. If an underfill material is used, it is not possible to remove a defective semiconductor device from a substrate and replace the defective part with a functional part because the underfill material is typically a thermosetting material, i.e. one which is permanently rigid and cannot be softened or flowed.
Because existing technology has the disadvantages set forth above, a need exists for an improved semiconductor device, and more specifically for an improved flip chip semiconductor device and method for making the same which can be burned-in in a manner which is suitable for a manufacturing environment, which does not require a semiconductor die to have an additional layer of metallization beyond the metallization used for internal circuitry, and which has the ability to be reworked without compromising thermal and mechanical performance. Furthermore, it is desirable for such a device to be able to be fabricated cost-effectively.